Film extrusion process for producing thermoplastic film

ABSTRACT

An apparatus for producing a thermoplastic film having low-birefringent, low stress and at least one polished surface for optical media applications as well as low stress film for non-optical applications using a continuous extrusion process. The apparatus consists of a novel calendering roll in a calendering roll stack wherein, at least one roll consists of an inner steel shell, a resilient covering over the inner steel roll and a multi-layer metal sleeve outer covering of at least two layers. The metal sleeve preferably consists of three layers. The inner layer of the multi-layer outer sleeve is preferably nickel, the intermediate layer is preferably copper and the outer layer is preferably chrome with a highly polished surface. The film produced by the process has a retardation value (birefringence times thickness) of less than about 100 nanometers and a surface roughness of less than about 4 microinches. The process is a continuous extrusion process for producing such film or sheet and does not require any further finishing operations.

BACKGROUND OF THE INVENTION

The invention relates to an apparatus for producing low-birefringentand/or low stress plastic film or sheet having a high surface-polish andis suitable for optical media applications as well as low stress filmfor non optical applications either one using a continuous extrusionprocess. Optical media applications include such items as compact discs(CD), digital video discs (DVD), liquid crystal displays (LCD) or anyother optical media applications which require a transparent substratewith low birefringence low stress and a high surface-polish. Non opticalapplications using low stress film or sheet for use in such applicationsas automobile dash board overlays or other uses for opaque film or sheetwhich require tight graphics registration. Birefringence is notmeasurable in opaque film or sheet.

More particularly, this invention relates to a particular calendering orprocess finishing roll stack wherein the structure of at least one ofthe finishing rolls is comprised of an inner steel shell, a resilientcovering thereover and a multi layer metal sleeve outer covering. Filmor sheet produced using the roll structure of this invention haslow-birefringence, low stress and is highly polished on at least onesurface i.e. a surface having a low roughness of 4 microinches or lesswhich film or sheet is suitable for optical media applications or opaquefilm or sheet for such other non optical applications. Such film orsheet is produced in a one step continuous extrusion process.

Currently, polycarbonate is used as the polymeric material for opticalmedia applications such as CD's and are made by injecting molding. Theprocess is relatively slow and expensive. In addition, it is difficultto produce CD's of very low-birefringence which will be required toreach higher data densities in the future. CD's currently produced todayhave a retardation value of 25-30 nm. (nanometers), which isbirefringence times thickness. Stress and birefringence are inherent ininjection molding CD's because the melt starts to solidify on the insidemold wall as the mold is filling, and then additional melt is forcedinto the mold cavity to compensate for shrinkage of the disc as itsolidifies. In opaque film or sheet, birefringence is not measurable butlow stress is wanted for applications in vehicles, computer housings,etc. that require tight (0.4 mm/MAX) graphics registration.

Birefringence is defined as the difference between the refractiveindices along two perpendicular directions as measured with polarizedlight along these directions. It results from molecular orientation, andthe measurement of birefringence is the most common method ofcharacterising polymer orientation. It is determined by measurement ofthe retardation distance by either a compensation or a transmissionmethod. Positive birefringence results when the principal optic axislies along the chain; negative birefringence when transverse to thechain. In Cartesian coordinates there are three birefringences, twobeing independent. Thus Δxy=n_(x)−n_(y), the differences in refractiveindices along the x and y axes. Uniaxial orientation only requires oneof these to describe the orientation. Therefore, in order to obtain auniform homogeneous polycarbonate, the lower the birefringence (thedifferences between the refractive indices) the more homogeneous thepolymer composition of the product and thus the more uniform propertiesof the product. This is critical, particular on CD's, DVD's or LCDwherein the Laser read out must have minimal or zero distortion. Thelower birefringence, the less is the variation in polymer homogeninityand Laser distortion.

Another parameter for optical materials is Cg which is thestress-optical coefficient of material in the glassy state. It can bemeasured with a molded part such as a small bar or disc. Birefringencecan be measured by the method described above. When a stress is appliedto the bar, the birefringence will change by an amount B. Thestress-optical coefficient, which has units of Brewsters, is given by:

B=Cg δ

The stress-optical coefficient (Cg) should be less than or equal toabout 70 Brewsters.

Improvements in optical data storage media, including increased datastorage density, are highly desirable, and achievement of suchimprovements is expected to improve well established and new computertechnology such as read only (ROM), write once, rewritable, digitalversatile and magneto-optical (MO) disks.

In the case of CD ROM technology, the information to be read isimprinted directly into a moldable, transparent plastic material, suchas bisphenol A (BPA) polycarbonate. The information is stored in theform of shallow pits embossed in a polymer surface. The surface iscoated with a reflective metallic film, and the digital information,represented by the position and length of the pits, is read opticallywith a focused low power (5 mW) laser beam. The user can only extractinformation (digital data) from the disk without changing or adding anydata. Thus, it is possible to “read” but not to “write” or “erase”information.

The operating principle is a write once read many (WORM) drive is to usea focused laser beam (20-40 mW) to make a permanent mark on a thin filmon a disk. The information is then read out as a change in the opticalproperties of the disk, e.g., reflectivity or absorbance. These changescan take various forms: “hole burning” is the removal of material,typically a thin film of tellurium, by evaporation, melting or spalling(sometimes referred to as laser ablation); bubble or pit formationinvolves deformation of the surface, usually of a polymer overcoat of ametal reflector.

Although the CD-ROM and WORM formats have been successfully developedand are well suited for particular applications, the computer industryis focusing on erasable media for optical storage (EODs). There are twotypes of EODs: phase change (PC) and magneto-optic (MO).

Generally, amorphous materials are used for MO storage and have adistinct advantage in MO storage as they do not suffer from “grainnoise”, spurious variations in the plane of polarization of reflectedlight caused by randomness in the orientation of grains in apolycrystalline film. Bits are written by heating above the Curie point,T_(C), and cooling in the presence of a magnetic field, a process knownas thermomagnetic writing. In the phase-change material, information isstored in regions that are different phases, typically amorphous andcrystalline. The film is initially crystallized by heating it above thecrystallization temperature. In most of these materials, thecrystallization temperature is close to the glass transitiontemperature. When the film is heated with a short, high power focusedlaser pulse, the film can be melted and quenched to the amorphous state.The amorphized spot can represent a digital “1” or a bit of information.The information is read by scanning it with the same laser, set at alower power, and monitoring the reflectivity.

In the case of WORM and EOD technology, the recording layer is separatedfrom the environment by a transparent, non-interfering shielding layer.Materials selected for such “read through” optical data storageapplications must have outstanding physical properties, such asmoldability, ductility, a level of robustness compatible with particularuse, resistance to deformation when exposed to high heat or highhumidity, either alone or in combination. The materials should alsointerfere minimally with the passage of laser light through the mediumwhen information is being retrieved from or added to the storage device.

As data storage densities are increased in optical data storage media toaccommodate newer technologies, such as DVD and higher density datadisks for short or long term data archives, the design requirements forthe transparent plastic component of the optical data storage deviceshave become increasingly stringent. Materials displaying lowerbirefringence at current, and in the future progressively shorter“reading and writing” wavelengths have been the object of intenseefforts in the field of optical data storage devices.

Birefringence in an article molded from polymeric material is related toorientation and deformation of its constituent polymer chains.Birefringence has several sources, including the structure and physicalproperties of the polymer material, the degree of molecular orientationin the polymer material and thermal stresses in the processed polymermaterial. For example, the birefringence of a molded optical article isdetermined, in part, by the molecular structure of its constituentpolymer and the processing conditions, such as the forces applied duringmold filling and cooling, used in its fabrication which can createthermal stresses and orientation of the polymer chains.

The observed birefringence of a disk is therefore determined by themolecular structure, which determines the intrinsic birefringence, andthe processing conditions, which can create thermal stresses andorientation of the polymer chains. Specifically, the observedbirefringence is typically a function of the intrinsic birefringence andthe birefringence introduced upon molding articles, such as opticaldisks. The observed birefringence of an optical disk is typicallyquantified using a measurement termed “in-plane birefringence” or IBR,which is described more fully below.

For a molded optical disk, the IBR is defined as:

IBR=(n _(r) −n _(θ))d=Δn _(rθ) d  (3)

where n_(r) and n_(θ) are the refractive indices along the r and θcylindrical axes of the disk; n_(r) is the index of refraction seen by alight beam polarized along the radial direction, and n_(θ) is the indexof refraction for light polarized azimuthally to the plane of the disk.The thickness of the disk is given by d. The IBR governs the defocusingmargin, and reduction of IBR will lead to the alleviation of problemswhich are not correctable mechanically. IBR is a property of thefinished optical disk. It is formally called a “retardation” and hasunits of nanometers.

In applications requiring higher storage density, such as DVD recordableand rewritable material, the properties of low birefringence and lowwater absorption in the polymer material from which the optical articleis fabricated become even more critical. In order to achieve higher datastorage density, low birefringence is necessary so as to minimallyinterfere with the laser beam as it passes through the optical article,for example a compact disk.

Materials for DVD recordable and rewritable material require lowin-plane birefringence, in particular preferably less than about +/−40nm single pass; excellent replication of the grooved structure, inparticular greater than about 90% of stamper; and reduced water uptakeas compared to BPA polycarbonate.

Another critical property needed for high data storage densityapplications, in particular DVD recordable and rewritable material, isdisk flatness. The disk flatness is dependent upon the flatness of thepolycarbonate substrate immediately after the injection molding processas well as the dimensional stability of the substrate upon exposure tohigh humidity environments. It is known that excessive moistureabsorption results in disk skewing which in turn leads to reducedreliability. Since the bulk of the disk is comprised of the polymermaterial, the flatness of the disk depends on the low water solubilityand low rate of water diffusion into the polymeric material. Inaddition, the polymer should be easily processed in order to producthigh quality disks through injection molding.

There is a distinct economic advantage of producing said film and sheetfor discs for optical media applications via a continuous film extrusionprocess, whereby a continuous plastic web of 4-8 feet wide could beproduced at speeds of 10-60 feet/minute from which discs could be cutout. Extrusion casting, where a melt is extruded through a slot die anddeposited on a polished metal roller to solidify, can producelow-birefringence films but the top surface of the film is not smoothenough. Extrusion calendering, on the other hand, whereby a secondpolished metal roll is added to form a nip between the two rolls tosqueeze the plastic on both sides as is solidifies, is widely used toproduce very uniform and smooth-surface films. However, the flow in thenip between rigid rolls induces very high stresses and such films haveretardation values of hundreds to thousands of nanometers. A resilientelastomeric cover has been put on one of the rolls to produce texturedfilms that have lower stress, but the texture is unacceptable foroptical media applications.

U.S. Pat. No. 3,756,760 teaches the use of a single metal outer sleeveof nickel over a rubber-covered roller to accommodate and smooth thenon-uniformity of the extrudate from an extrusion die upon deliveringmelt to the calendering nip. It does not disclose how to use this tocontrol stress in the film and birefringence. In addition, such a sleeveis too fragile to be of practical use.

U.S. Pat. No. 5,076,987 discloses producing optical quality extrusionfilm by calendering the film between a ground elastic roller and a highgloss steel roller to produce a film having a high gloss surface and amatte surface, or producing a film having a high gloss on both surfaces,by coating the matte surface.

U.S. Pat. No. 5,149,481 discloses extruding a sheet or film into theroll gap of a smoothed upper roll and a lower roll wherein thetemperature of the upper roll is below the glass transition temperatureof the plastic and the lower roll is maintained at a temperature in theplastic state domain of the plastic sheet or film.

U.S. Pat. No. 5,242,742 is similar to U.S. Pat. No. 5,149,481 exceptthat it discloses a sheet or film having a birefringence of less than 50nm and preferably less than 20 nm.

U.S. Pat. No. 4,925,379 discloses a process for producing a plasticsheet, wherein at least one layer is a polyurethane layer, by extrusionand pressing at a temperature higher than the softening point of thepolyurethane.

U.S. Pat. No. 5,286,436 is a division of U.S. Pat. No. 5,242,742 andclaims a sheet or film having a birefringence equal to or less than 50nm, a low surface roughness and low variation in thickness.

All of the above references do not disclose or teach the particularfinishing roll of the instant invention or a low birefringence, lowstress highly polished film or sheet by a continuous extrusion process.

Accordingly, it is an object of this invention to produce a lowbirefringence, low stress, highly surface polished thermoplastic film orsheet on at least one surface thereof.

Another object of this invention is to provide means for producing a lowbirefringence, low stress, highly surface polished thermoplastic film orsheet on at least one surface thereof.

Still another object of this invention is provide a low birefringencetransparent film or sheet suitable for optical media applications.

Yet, another object of the invention is to provide a one step continuousextrusion process for producing a low birefringence, low stress, highlysurface polished transparent thermoplastic film or sheet.

These and other objects will become apparent from the followingdescription of this invention.

SUMMARY OF THE INVENTION

The present invention is directed to products, apparatus and process forpreparing thermoplastic film or sheet for optical media applications ornon optical applications in the case of opaque film or sheet. Theapparatus for producing a low birefringence, highly surface polishedthermoplastic film or sheet comprises a calendering roll stack whereinat least one roll being of a particular novel construction in order toproduce the product of this invention as well as a one step continuousextrusion process for producing the product of this invention whichproduct is a low birefringence, low stress, highly polished transparentfilm or sheet or a low stress highly polished opaque film or sheet.

It has been surprisingly discovered that the novel structure of thecalendering roll of this invention comprises at least a three componentcalendering roll structure comprised of an inner metal shell, anintermediate resilient elastomeric cover over the inner metal shell anda multi-layer metal sleeve outer covering. The multi-layer metal sleeveouter covering comprises at least two layers and preferably a threelayer metal sleeve outer covering. The novel calendering roll of thisinvention and an opposing calendering roll form a calendering nip or gapthrough which thermoplastic film or sheet is continuously extruded. Thisprocess is also known as a continuous film or sheet extrusion process.As used herein, the terms “film” and “sheet” are used interchangeablyand refer to thermoplastic material having a final thickness of about0.001 to about 0.060 inches but may be thicker, if so desired, dependingon the final application.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an extruder illustrating the extrusion ofa thermoplastic melt through a calendering finishing roll stack forproducing the thermoplastic film of this invention.

FIG. 2 is an enlarged sectional schematic section A-A′ of the novelcalendering roll of this invention.

FIG. 3 is an enlarged sectional schematic section B-B′ of the componentparts of the novel calendering roll of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention comprises products, and process apparatus forproducing thermoplastic film particularly for transparent or translucentoptical media applications wherein the product is a film of lowbirefringence, low stress and a highly polished surface on at least onesurface thereof having a low roughness of less than about 4 microinches.The process comprises the steps of extruding a thermoplastic melt,passing the hot thermoplastic melt through the nip or gap of calenderingrolls wherein at least one calendering roll is preferably the finishingroll of this invention, and the opposing calendering roll maybe astandard chrome plated steel roll, cooling the thermoplastic film tobelow its glass transition temperature and forwarding the cooledthermoplastic film, preferably, to a winder for winding the film in aroll. The process and apparatus are also suitable for producing lowstress non optical opaque film for other applications wherein such filmis used in vehicles, computer housings, telecommunications, etc.

While the invention is described as at least one finishing roll of acalendering roll stack, the calendering roll stack may be comprised ofat least two of the novel finishing calendering rolls of this invention.The finishing roll of this invention is a multi component structurecomprised of an inner steel shell, an intermediate resilient elastomericcovering over the inner steel shell and a multi layer metal sleeve outercovering. The resilient covering may be any type of resilient coveringbut is typically a silicone based rubber of about ⅛ inch to about 1 inchin thickness nay have a hardness of about 50 to about 150 durometer(Shore A), or the resilient covering may be an EPDM (ethylene propylenediamine monomer) based rubber. The resilient intermediate layer shouldbe resilient to the extent that the resilient cover allows deformationof the outer multi-layer metal sleeve covering in order to greatlyreduce the flow-induced stress which would lead to high birefringenceand high stress thermoplastic film. The resilient covering shouldpreferably have a maximum temperature rating for continuous use of about600° F.

The outer multi-layer metal sleeve is typically about 0.005 inches toabout 0.020 inches in total thickness which permits flexing to match theresilient intermediate cover underneath. The outer multi-layer metalsleeve may be a two or more layer sleeve but is preferably at least athree layer sleeve. The outer layer of the preferably three-layer outercovering preferably comprises a high density chrome outer layer of about0.0002 to about 0.002 inches thick that can be polished to a smoothfinish preferably a roughness of less than 4 microinches and moreparticularly less than 2 microinches and even more particularly lessthan 1 mircro-inch roughness. Chrome plating and chromium plating areused interchangeably to describe a thin layer of chromium which may bedeposited by electrolysis on an oxidizable metal. The chrome layerthickness of less than 0.002 inch reduces the strain during flexing sothe chrome will not crack and fail. The inner layer of the metalmulti-layer outer metal sleeve is typically nickel or nickel based alloysuch as Monel 400, which is 67 weight % nickel and 30 weight % copper,of about 0.002 to about 0.010 inches thick for mechanical integrity ofthe sleeve during it's manufacturing, handling and installation on theroller and preferably has a hardness rating of about 220 Vickers. Nickelis preferred for the inner layer of the multi-layer outer sleeve becauseof its low porosity and smooth surface. The middle layer or intermediatelayer between the outer and inner layer thereof is typically copper ofabout 0.005 to about 0.020 inches thick and preferably about 0.001 toabout 0.005 inches thick, which is relatively soft, ductile and flexiblewhich also greatly reduces the strain on the chrome layer during flexingand prevents failure. It also exhibits good adhesion to both chrome andnickel. Nickel or nickel based alloys are employed as they are lessbrittle than chrome at room temperature. The novel roller of thisinvention provides both resiliency and surface polish at the same timeand thus is able to produce low birefringence, low stress, highlypolished film suitable for optical media applications.

A two layer outer metal sleeve may also be employed wherein the innerlayer is preferably a nickel based alloy and the outer layer ispreferably chrome. The inner layer would have thickness of about 0.014inches and the outer layer of about 0.0015 inches thick.

The thermoplastic material that may be employed in producing the productof this invention, includes without limitation, aromatic polycarbonate,copolymers of an aromatic polycarbonate such as polyester carbonatecopolymer, blends thereof, and blends thereof with other polymersdepending on the end use application. Preferably the thermoplasticmaterial is an aromatic polycarbonate resin and examples ofpolycarbonate resins are described in U.S. Pat. No. 4,351,920 which isincorporated herein by reference. They are obtained by the reaction ofan aromatic dihydroxy compound with a carbonyl chloride. Otherpolycarbonate resins may be obtained by the reaction of an aromaticdihydroxy compound with a carbonate precursor such as a diarylcarbonate. A preferred aromatic dihydroxy compound is 2,2-bis(4-hydroxyphenyl) propane (i.e. Bisphenol-A). The polyester carbonate is obtainedby the reaction of a dihydroxy phenol, a carbonate precursor anddicarboxylic acid such as terephthalic acid or isophthalic acid or amixture of terephthalic and isophthalic acid. Optionally, an amount of aglycol may also be used as a reactant.

The transparent or translucent film produced by the practice of theinvention has a low birefringence, a low stress and is highly polishedon at least one surface thereof. The retardation value of the film i.e.birefringence times thickness is 100 nm or less and is preferably lessthan about 20 nm and more particularly less than about 15 nm. If theretardation value is over 100 nm the film has a high birefringence and ahigh stress value both of which are not wanted in the final film productof this invention since it would not be suitable for transparent ortranslucent optical applications and for opaque films where high stressis unacceptable. The surface of the highly polished thermoplastic filmis less than about 4 microinches in roughness and preferably about 0.5to about 2.0 microinchs in roughness. The transparent film also has lessthan 1% haze.

Opaque films produced in the practice of this invention are low stressfilms having a polished surface on at least one surface thereof. Sincethe films are opaque, birefringence is not measurable. However, lowstress film is greatly desired for such applications as recited above,namely, on road and off road vehicles, computer housings,telecommunication equipment and such other application requiring lowstress opaque film for graphics registration.

The process of producing the film of this invention comprises feeding athermoplastic resin to a screw extruder, heating the resin to above itsglass transition temperature (Tg) producing a viscous melt ofthermoplastic resin, passing the viscous thermoplastic melt underpressure through the die orifice of the extruder which die orifice isgenerally a slot forming a continuous film of molten thermoplastic resin(extrudate), passing the extrudate through the nip or gap of a pair of acalendering roll system to form the finished film.

Preferably one roll of the calendering rolls is the multi component rollof this invention and is comprised of an inner steel shell, a resilientcovering thereover and a multi-layer metal sleeve outer covering overthe resilient covering.

While the calendering rolls are shown in a vertical stackingconfiguration, the calendering rolls may lie in a horizontal plane or ina plane at any angle of from 90° vertical to 0° horizontal. The angle ofthe plane at which the calendering rolls lie is not critical to theinvention. The criticality of this invention lies in the structure ofthe finishing roll of this invention.

FIG. 1 is a schematic drawing of the continuous process of thisinvention and the apparatus employed herein illustrating extruder 2 withfeed hopper 4 and through which thermoplastic resin 6 is fed to barrel 8of extruder 2. The extruder is heated to a temperature sufficient tomelt thermoplastic resin 6 which temperature is above the glasstransition temperature (Tg) of the thermoplastic resin. The meltedthermoplastic resin is advanced through extruder 2 by screw 9 to filmdie orifice 10. The extruded thermoplastic melt 11 is passed through nipor gap 12 formed by rolls 14 and 16, around roll 18 through pull rolls20. Finished film 21, having a low birefringence, low stress and ahighly polished surface, is wound on to roll core 24, by winder 22.

FIG. 2 is an enlarged cross sectional view A-A′ of roll 14 which is thenovel finishing roll of this invention. Finishing roll 14 comprisesinner steel shell 30, intermediate resilient cover 28 and outermulti-layer sleeve 26.

FIG. 3 is cross section of B-B′ illustrating the three componentstructure of finishing roll 14 and in particular the multi-layer outermetal sleeve. While the multi-layer outer metal sleeve is illustratedand described as a three layer outer sleeve, it may consist of less thanthree or more than three layers. Inner steel shell 30 is covered byintermediate resilient cover 28, which is covered by outer multi-layersleeve 26. Outer multi-layer sleeve 26 is illustrated as consisting ofthree layers, inner layer 36, which may be nickel, intermediate layer34, which may be copper, and outer layer 32, which may be chrome.

The following example is provided merely to show one skilled in the arthow to apply the principals of this invention as discussed herein. Thisexample is not intended to limit the scope of the claims appended tothis invention.

EXAMPLE I

A 55-inch long by 12-inch diameter calendering roll was constructedaccording to the present invention. The resilient cover was siliconerubber, ⅜-inch thick and 70 durometer hardness. The intermediate outermulti-layer sleeve was 0.005 inches of nickel, 0.009 inches of copper,and 0.0015 inches of chrome. The chrome was polished to approximately2-4 microinch surface roughness. The inner shell of the roll was steel.The plastic extruded was polycarbonate of a melt flow index ofapproximately 6, extruded by a conventional single-screw extruderthrough a conventional film die. Line speed was 19 fpm, nip load was 100pli, roll temperature 245° f. and pull roll amps 1.6 (proportional toweb tension). Approximately 25,000 feet of 4-ft wide 0.010 inch thickfilm was produced having an average retardation value of 14 nm and astandard deviation of 1.8 nm. For comparison, typical samples ofcommercial “high quality” polish, such as polished polycarbonate 0.010inches thick, produced by the conventional calendering process haveretardations of 325-500 nm.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation; the spiritan scope of the present invention being limited only in terms of theappended claims.

What is claimed:
 1. A continuous film extrusion process for producing atransparent or translucent thermoplastic film for optical mediaapplications having a low birefringence, low stress and wherein at leastone surface of the film has a roughness of less than about 4 microinchesand a retardation value of less than about 100 nanometers, which processcomprises: extruding a molten thermoplastic film, and passing the moltenthermoplastic film through an opening between two opposing calendaringrolls wherein at least one calendaring roll is a finishing roll andsubsequently cooling the hot thermoplastic film to a temperature belowits solidification temperature of the thermoplastic film, said finishingroll is a multi component structure comprised of an inner steel shell,an intermediate resilient covering over the inner steel and amulti-layer metal sleeve outer covering comprised of at least twolayers.
 2. The process of claim 1, wherein the cooled thermoplastic filmhas a thickness of about 0.001 to about 0.060 inches.
 3. The process ofclaim 1 wherein at least one surface of the thermoplastic film has aroughness of about 0.5 to about 2.0 microinches.
 4. The process of claim1 wherein the 2 opposing calendaring rolls are finishing rolls used toproduce a thermoplastic film of low birefringence, low stress andwherein bath surfaces of the thermoplastic film have a roughness of lessthan about 4 microinches.
 5. The process of claim 4 wherein bothsurfaces of the thermoplastic film have a roughness of about 0.5 toabout 2.0 microinches.
 6. The process of claim 1 wherein the multi-layermetal sleeve outer covering has a total thickness of about 0.005 inchesto about 0.020 inches.
 7. The process of claim 1 wherein the multi-layermetal sleeve outer covering further comprises three-layers with a chromeouter layer having a thickness of about 0.0002 inches to about 0.002inches.
 8. The process of claim 7 wherein the multi-layer metal sleeveouter covering further comprises an inner layer having a thickness ofabout 0.002 inches to about 0.010 inches and comprising a materialselected from the group of nickel and a nickel based alloy.
 9. Theprocess or claim 8 wherein the multi-layer metal sleeve outer coveringfurther comprises a middle layer disposed between the outer layer andthe inner layer, and wherein the middle layer comprises copper and has athickness of about 0.005 inches to about 0.020 inches.
 10. A continuousfilm extrusion process for producing a thermoplastic film having lowstress and wherein at least one surface or the film has a roughness ofless than about 4 microinches and a retardation value of less than about100 nanometers, which process comprises: extruding a moltenthermoplastic him, and passing the molten thermoplastic film through anopening between two opposing calendaring rolls wherein at least onecalendaring roll is a finishing roll and subsequently cooling the hotthermoplastic film to a temperature below its solidification temperatureof the thermoplastic film, said finishing roll is a multi componentstructure comprised of an inner steel shall, arm intermediate resilientcoveting over the inner steel and a multi-layer metal sleeve outercovering comprised of at least two layers.
 11. The process of claim 10,wherein the cooled thermoplastic flint has a thickness of about 0.001 toabout 0.060 inches.
 12. The process of claim 10, wherein at least onesurface of the thermoplastic film has a roughness of about 0.5 to about2.0 microinches.